1. Field of the Invention
The present invention relates to a torque measuring apparatus and, more particularly, to an improved torque measuring apparatus for measuring the torque which is transmitted through a rotary magnetic member by magnetostriction in a noncontacting state.
It is necessary to measure transmitted torque accurately and easily in various rotary driving apparatus in order to analyze and grasp the driving apparatus in various industrial fields.
As such rotary driving apparatus various motors are generally known, and they are utilized in almost all industrial fields, in particular, as vehicle engines, electric motors of electric automobiles and industrial motors. In order to analyze the driving state of such a rotary driving apparatus, it is necessary to accurately measure its torque as well as its rotational frequency.
Especially, in vehicle engines or the like, measurement of transmitted torque of various driving systems such as an engine transmission, propeller shaft and differential gear by means of the real-time measurement is utilized for controlling the ignition timing, the amount of fuel injection, the transmission timing, and the transmission ratio. The optimum control of these factors can improve the specific fuel consumption of vehicles, drivability and the like.
Such rotary driving apparatus have some poles which are characteristic of rotary systems; for example, reciprocating engines have some cylinders and electric motors have some magnetic poles. Therefore, the torque output from a rotary driving apparatus has a torque inflection point at which the magnitude of the torque rapidly changes in correspondence with each pole, and in most cases it is necessary to measure the torque as a mean value at each intervals between the inflection points.
In a four-cylinder reciprocating gasoline engine, for example, the torque output of the engine is the synthesized output of the four cylinders in four cycles of suction, compression, combustion and exhaust stroke. In one cylinder, it outputs a large positive torque in the combustion stroke, and in the other strokes torque is consumed and the output takes a small negative value.
In the four cylinders constituting the reciprocating engine, the power stroke is conducted in the order of the No. 1 cylinder, No. 3 cylinder, No. 4 cylinder and No. 2 cylinder. When the rotational angle at the top dead center of the No. 1 cylinder is set at 0 degree as a reference position, the No. 1, 3, 4 and 2 cylinders conduct combustion strokes at intervals of 0 to 180 degrees, 180 to 360 degrees, 360 to 540 degrees, and 540 to 720 degrees, respectively, and output large positive torques.
If it is possible to measure the torque of the engine itself or each torque of various driving force transmission systems such as a transmission, propeller shaft and differential gear as a mean value at each interval between the inflection points, it is possible to control the ignition timing and the amount of fuel injection separately for each cylinder of the engine and, hence, it is possible to improve the fuel efficiency and drivability.
Furthermore, in electric motors and other industrial motors, measurement of torques as a mean value at each interval between inflection points enables the optimum control and diagnosis of the rotary driving system.
2. Description of the Prior Art
When torque is transmitted through a rotary driving system, strain is produced in the rotary members such as a shaft, flywheel and a clutch disc in correspondence with the transmitted torque. Therefore, it is possible to measure the transmitted torque by detecting the amount of strain.
A torque measuring apparatus for detecting the amount of strain produced in a rotary member utilizing a magnetostrictive effect is known for this purpose. In this apparatus, a part of a rotary member which transmits torque is made of a ferromagnetic material and the magnetostrictive amount of the rotary magnetic member is detected by a magnetic sensor so as to measure the transmitted torque in a noncontacting state.
The detection signal of the magnetic sensor is output as a sum of a component which depends upon the transmitted torque and an offset component which does not depend upon the torque. Therefore, it is necessary to subtract the offset component from the output of the magnetic sensor in order to measure the exact torque.
The magnitude of the offset component varies in accordance with the rotation of the rotary magnetic member. In a conventional measuring apparatus, however, the magnitude of the offset component is assumed to be constant, and a predetermined constant value is subtracted from the output of the magnetic sensor as the offset component.
As a result, in the conventional measuring apparatus it is impossible to measure the exact torque and, in particular, when an offset component greatly varies, the measured value becomes very inaccurate.
FIGS. 8 and 9 are schematic views of a torque measuring apparatus provided in the torque transmission mechanism of a vehicle engine, wherein FIG. 8 is a schematic side elevational view thereof and FIG. 9 is a schematic sectional view, taken along the line IX--IX.
As is known, the torque produced in the engine is transmitted to a flywheel (not shown) through a rotary shaft 10, and is transmitted to the transmission side through a clutch disc which comes into frictional contact with the flywheel.
When torque is transmitted in this manner, anisotropy of strain which is proportional to the magnitude of the transmitted torque is generated on the rotary shaft 10 and the rotary disc (not shown) such as the clutch disc and the flywheel. If the torque transmission mechanism is made of a ferromagnetic material, it is possible to measure the transmitted engine torque by magnetically detecting the magnitude of the generated anisotropy of strain in a noncontacting state utilizing the magnetostrictive effect.
In the torque measuring apparatus, therefore, the rotary shaft 10 or the flywheel themselves is made of a ferromagnetic material, or a ferromagnetic material is attached to the surface of the rotary shaft 10 or the flywheel, and a magnetic sensor 12 is opposed to the rotary magnetic member formed in this manner with a predetermined space therebetween.
The magnetic sensor 12 used here is composed of a U-shaped exciting core 14 which is arranged in parallel to the rotary shaft 10, and a U-shaped pickup core 18 which is disposed inside the exciting core 14 such as to be orthogonal thereto, with an exciting coil 16 wound around the exciting core 14, and a pickup coil 20 wound around the pickup core 18.
FIG. 10 shows an electric circuit formed using the magnetic sensor 12. To the exciting coil 16 a sine-wave voltage is applied from AC power source 22 for alternate magnetization of the rotary shaft 10 which is opposed to the magnetic sensor 12.
If torque is transmitted through the rotary shaft 10, stress is produced in the rotary shaft 10 and a magnetic flux component is generated in the direction orthogonal to the exciting direction by virtue of the magnetostrictive effect. The magnetic flux component is detected by the pickup coil 20 as an induced voltage. The induced voltage is amplified by an AC amplifier 24 and is thereafter rectified by a rectifier 26. An offset component which is contained in the detection signal is treated as a constant value, and subtracted from the rectified signal by an offset subtractor 28. The subtracted signal is thereafter output as a signal which is proportional to the torque. The offset component contained in the detection signal of the magnetic sensor 12, however, varies with the rotation of the shaft and is not constant, actually. Nevertheless, in a conventional measuring apparatus, since the magnitude of the offset component is regarded as constant, the value of the offset signal which has to be subtracted by the subtractor 28 from the rectified signal is different from the value of the actual offset component, so that it is impossible to measure the torque accurately.
When the magnitude of the offset component varies to a great extent with the shaft rotation, a set value of the offset signal and the values of the actual offset component are so greatly different that the measured value is very inaccurate.
FIG. 11 shows the signal wave form of the voltage Ao which is output from the rectifier 26 when a constant torque is applied to the rotary shaft and the transmitted torque is measured. The wave form Si represents the voltage wave form when the torque of 100 Nm is transmitted, while Soi represents the voltage wave form when the transmitted torque is set at 0 Nm, namely, Soi means the offset voltage wave form.
As is obvious from FIG. 11, the signal output from the rectifier 26 contains an offset component which varies in magnitude with the shaft rotation and, in addition, the magnitude of the offset component is greatly different at the respective intervals between the inflection points of 0 to 180 degrees, 180 to 360 degrees, 360 to 540 degrees, and 540 to 720 degrees, which correspond to respective cylinders from the first to fourth cylinders.
The present inventor analyzed the causes for generating a offset component in order to remove it, and found that the following items are the causes.
(a) Residual strain of the rotary magnetic member (the rotary shaft 10 in FIGS. 8 and 9) which is opposed to the magnetic sensor.
(b) Magnetic nonuniformity of the rotary magnetic member.
(c) Variation of the relative positional relationship between the magnetic sensor and the rotary magnetic member.
(d) Crosstalk of an exciting signal produced by the magnetic sensor.
(e) Offset voltage generated by treatment of the electric circuit.
Among these, the offset components caused by the items (d) and (e) have constant values, and the offset component caused by the item (c) can be made approximately constant by appropriately setting the arrangement of the magnetic sensor 12 with respect to the rotary magnetic member.
In contrast, in order to make the magnitudes of the offset components caused by the items (a) and (b) constant, it is necessary to make both the mechanical residual strain of the rotary magnetic member and the magnetic properties such as easy magnetization direction, permeability and saturated magnetic flux density uniform along the shaft circumference.
It is, however, actually impossible to make a rotary magnetic member which has circumferentially uniform mechanical properties and magnetic properties while the mechanical strength of the shaft us enought to transmit torques through one. Hence, it is impossible to make the offset components caused by the items (a) and (b) constant.
As shown by the wave form Si in FIG. 11, the signal Ao which is output from the rectifier 26 greatly varies due to the offset component even if the transmitted torque is constant. Therefore, it will be understood that the measurement of the exact torque is impossible simply by subtracting a constant value from the output Ao of the rectifier 26 as the offset signal component, and the output Eo of the subtractor 28 pulsates with the shaft rotation, as shown in FIG. 12.
In this case, the mean value Soi of the offset component which is output while the rotary magnetic member rotates a predetermined number of times, e.g., 10 to 100 times is obtained in advance and this value Soi is set on the subtractor 28 as the offset data. The offset mean value Soi is subsequently subtracted from the output signal Ao of the rectifier 24, and the mean value of the subtracted value Eo is obtained every time the rotary magnetic member rotates a predetermined number of times, and the transmitted torque is measured on the basis of the mean value Eo.
In spite of this countermeasure, the torque transmitted through the rotary magnetic member is measured merely as the mean value for every predetermined number of rotations of the rotary magnetic member and the real-time measurement of the transmitted torque with high resolution is impossible. Especially, when the transmitted torque frequently varies, it is impossible to measure it accurately.
In addition, such a conventional measuring apparatus cannot measure interval torque at every interval between the inflection points simultaneously with high resolution, and an effective countermeasure has been in demand.